Why does metal corrode, and how do surface preparation and finishes such as painting, plating, galvanising and anodising protect and improve a metal product?
Explain corrosion of metals and describe the surface preparation and finishing processes used to protect and decorate metal, including painting, electroplating, galvanising, anodising and powder coating
A focused guide to metal finishing for HSC Industrial Technology Metal and Engineering. Why metals corrode, surface preparation, protective and decorative finishes including painting, electroplating, galvanising, anodising and powder coating, and selecting a finish.
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What this dot point is asking
Most metals corrode unless they are protected, so finishing is essential to make metal products durable and attractive. NESA expects you to explain why corrosion happens and to describe the surface preparation and finishing processes used to protect and decorate metal, such as painting, plating, galvanising, anodising and powder coating. This content is examined in the written paper and decides how well the metalwork in your Major Project survives in service.
Why metals corrode
Corrosion is the chemical reaction of a metal with its environment, most commonly with oxygen and moisture. When iron and steel corrode they form rust, an iron oxide that flakes off and exposes fresh metal, so rusting continues until the part is consumed. Salt and pollution speed it up, which is why marine and outdoor metalwork corrodes fastest. Some metals, such as aluminium and stainless steel, form a thin, tight oxide layer that protects the metal beneath, which is why they resist corrosion naturally. Understanding this explains why protective finishing is necessary.
Surface preparation
A finish only adheres to and protects a clean surface, so preparation comes first. Surfaces are cleaned of oil and grease (degreasing), and rust, scale and old coatings are removed by wire brushing, grinding, abrasive blasting or chemical pickling. The metal may then be primed or treated to help the finish bond. Skipping preparation is the commonest reason a finish lifts or fails early, so it is treated as half the job.
Protective and decorative finishes
- Painting: a primer plus a top coat forms a barrier that keeps oxygen and moisture off the metal and adds colour. Cheap and versatile, but it needs maintenance because chips let corrosion start.
- Electroplating: an electric current deposits a thin metal coating, such as chrome, nickel or zinc, for corrosion resistance and a decorative finish.
- Galvanising: coating steel with zinc. The zinc not only forms a barrier but also corrodes sacrificially in place of the steel, protecting it even if the coating is scratched, which is why galvanised steel lasts so well outdoors.
- Anodising: an electrochemical process that thickens the natural oxide layer on aluminium, giving a hard, durable, corrosion-resistant surface that can also be coloured.
- Powder coating: a dry powder is applied electrostatically and baked to a tough, even, attractive film, widely used for outdoor furniture, fencing and components.
Sacrificial protection
Galvanising shows an important principle: a more reactive metal can protect a less reactive one by corroding first. The zinc coating, and sacrificial anodes used on larger structures, are consumed in place of the steel. This explains why a scratched galvanised surface still resists rust while a scratched painted surface does not.
Selecting a finish
Match the finish to the metal, the environment and the look you want. Use galvanising or powder coating for steel exposed to weather, anodising for aluminium that needs a hard durable surface, electroplating for a decorative corrosion-resistant finish on fittings, and paint for cost-effective protection where it can be maintained. Always prepare the surface properly first, and state the corrosion risk and appearance that drive your choice when you justify it in the folio.
Exam-style practice questions
Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2023 HSC4 marksDescribe the process of pickling a corroded piece of metal. (Industrial Technology, Metal and Engineering Technologies, Section II)Show worked answer →
Pickling is a chemical surface preparation that removes rust, scale and oxides from corroded metal before a protective finish is applied. A 4 mark answer describes the full sequence.
Clean and degrease. First remove oil, grease and loose dirt so the acid can reach the corroded surface evenly.
Immerse in the pickling bath. The work is dipped in a dilute acid solution (commonly sulfuric or hydrochloric acid), often warmed, which dissolves the iron oxide (rust) and mill scale from the surface. An inhibitor is added so the acid attacks the scale rather than the sound metal beneath.
Control time and strength. The part is left only long enough to strip the corrosion; over-pickling etches and weakens the base metal.
Rinse and neutralise. The metal is rinsed thoroughly in clean water and neutralised so no acid remains, then dried. It is now a clean, chemically active surface ready for painting, plating or galvanising.
Markers reward naming the acid, stating that it dissolves the rust and scale, and including rinsing and neutralising. Mention WHS, acids are corrosive, so gloves, eye protection and ventilation are required.